U.S. Pat. Nos. 4,443,789 and 4,360,892 both of Enfield disclose examples of this type of keyboard.
U.S. Pat. No. 3,833,765, which requires sequential keying, discloses a typewriter capable of being produced in a pocket-sized version. The above patents are directed to keyboard entry data transmission systems, employing one-handed keyboards wherein single characters are uniquely defined by a plurality of key depressions.
The above mentioned patents represent two types of keying (see U.S. Pat. No. 4,344,069 for an explanation) used in one-handed typewriters. The first type of keying is chord keying in which a number of keys are pressed simultaneously; this combination uniquely defines a character. The second type of keying is sequential keying in which a number of keys are pressed in a sequential order that uniquely identifies a specific character.
U.S. Pat. No. 4,042,777 discloses various electronics packages or logic circuits which can be used in conjunction with one-handed keyboards. One-handed keyboards are useful for high-speed communications, making records and providing a means for the hearing impaired or other handicapped people to communicate.
Other designs of five-key keyboards include U.S. Pat. No. 647,853 and Swedish Pat. No. 189,742.
IBM Technical Disclosure Bulletin, Vol. 3, No. 6, dated Nov. 1960, page 16, discloses a 10-key keyboard operated by four fingers and a thumb in a hand cover. Movement of a finger or a thumb toward the center of the cover operates one set of keys, and movement away from the center operates a second set of keys.
A large variety of various types of keyboards and key switches are disclosed in International Patent Application WO 82/011345, including unusual switches at FIGS. 11 and 14 through 17.
Communication aids for the handicapped, employing keyboards, are disclosed in U.S. Pat. Nos. 3,507,376; 4,074,444; and 4,075,621.